5.3: Nervous System Flashcards
(49 cards)
Function of dendrites
Receive action potential from preceding/ relay/ intermediate/ connector neurones
Function of myelin sheath
Insulates axon causing saltatory conduction
Function of Schwann cells
Secrete myelin
Explain how depolarisation occurs at a synapse
Action potentials arrive at the synapse
Ca2+ ions enter synaptic knobs
Causes release of acetylcholine from synaptic bulbs
Attaches to receptors on the sarcolemma
Making it permeable to Na+ ions
Which diffuse in fibre causing depolarisation
Explain how is a resting potential generated:
Axon is impermeable to Na+ but permeable to K+; Na+ pumped out of axon to tissue fluid; more negative inside; inflow of K+ less than outflow of Na+ (3 Na out for 2 K in); ATP required for pumps
What is meant by a threshold stimulus?
The smallest stimulus that is capable of setting up an action potential
How are action potentials propagated along the axon during nerve impulse transmission:
Propagated by local currents; e- flow occurs at margins of depolarised (+) and resting/ repolarised (-) regions; these currents make the next bit of axon membrane permeable to Na+ and so the region of depolarisation spreads
Why do synaptic bulbs contain many mitochondria?
Provide ATP; to provide energy for combining of acetate/ choline to synthesise neurotransmitter; move vesicles (exocytosis)
How does the post-synaptic membrane become repolarised?
Acetylcholinesterase enzyme releases as soon as muscle is depolarised; this removes the acetylcholine from the receptors; by hydrolysing into acetate + choline; membrane of muscle reverts to being impermeable to sodium ions: resting potential restores
Distinguish between a motor neurone and a motor nerve:
Motor neurone: single cell running from CNS to effector organ;
Motor nerve is made of many motor neurones running side by side to effector(s); contain bundles of neurones grouped together in connective tissue/ collagen sheaths
Refractory period of a neurone:
The period which must elapse after a (first) stimulus before a second stimulus can produce a second action potential; axon must have repolarised almost completely before another potential can be set up
[book: Na voltage gated channels remain closed; so action potentials are unidirectional; no overlap]
Distinguish between spatial and temporal summation at synapses:
The depolarisation effect of discharging synaptic knobs is cumulative (= summation); in spatial summation several synaptic knobs discharge simultaneously (onto the post-synaptic membrane); whereas in temporal summation they discharge in rapid succession
All or nothing law:
If a stimulus is above its threshold value; it sets up a complete full sized action potential; larger stimuli don’t increase the size of action potential
Outline stages at resting state:
Membrane is impermeable to Na; Na pumped out by active transport/ higher conc maintained outside; K enter along electrical gradient; influx of K can’t catch up with out flux of Na so membrane is charged
Outline stages of depolarisation:
Stimulus causes membrane to become permeable to Na/ (voltage-gated) Na channels open; Na+ diffuse in the neurone (down electrochemical gradient); membrane potential becomes positive; positive feedback
Outline stages of repolarisation:
Na channels close; (voltage-gated) K channels open/ K+ move out of neurone; membrane potential becomes negative; positive feedback/ more K channels open
Outline stages of hyperpolarisation:
K ions continue to leave/ K channels slow to close; inside of cell becomes more negative than resting stage;
Outline how the first neurone communicates with the second neurone across the gap:
Neurotransmitter released from pre-synaptic membrane; diffuses across synaptic cleft; attaches to receptors of Na channels on post-synaptic membrane; neurotransmitter broken down in cleft
The relationship between strength of a stimulus and the resulting action potential:
Only stimuli that reach threshold value produce an action potential; all-or-nothing law: ap either occurs or not; so is same magnitude no matter the strength; strong stimulus produces many ap (in rapid succession)
Outline roles of synapses in the nervous system:
Allows neurones to communicate/ cell signalling;
ensure transmission in one direction; allows divergence/ convergence;
filter out low level stimuli;
prevent over stimulation;
allow many low level stimuli to be amplified;
presence of inhibitory/ excitatory synapses allow impulses to follow specific paths;
permits memory/ learning
Outline the importance of the junctions between neurones in the functioning of the nervous system:
Ensures movement of impulse/ action potential in one direction only; integration- one neurone can, connect to/ receive impulses from/ transmit impulses, many neurones;
allows summation;
filters out background/ low level stimuli- only stimulation strong enough will be passed on;
permits memory/ learning; acclimatisation;
prevents continuous stimulation of neurones;
2 types of synapses: excitatory inhibitory
Why is the Pacinian corpuscle described as a transducer?
Converts mechanical energy to electrical energy
Deformation of plasma membrane of the tip of the neurone causes the membrane to become more permeable to Na+ suggest why?
Increased pressure causes Na channels to open; (temporary) gaps appear between the phospholipids/ in the bilayer
Suggest an explanation for the fact that action potentials are not generated constantly whilst wearing clothes:
Na channels remain open/ resting potential not reestabilished; ions in the wrong place for movement across membrane
